Velocity dependence of the Penning and associative ionization cross sections of Ar atoms by He(23S) and He(21S) atoms

The velocity dependence of both the Penning and associative ionization cross section of Ar atoms by He(23S) and He(21S) metastable atoms, and of their sum is measured in the velocity range 1200-4500m/sec (30-400MeV) in a crossed-beam experiment by a time-of-flight technique. Characteristic cross-section ratios associative/total and singlet/triplet are reported along with the relative magnitude of the associative, Penning and total cross-sections. The measurements clearly show a difference in the reaction mechanisms involving He(23S) and He(21S). This difference appears not only in the ratio associative/total, whose magnitude reaches 19% and 38% for He(23S) and He(21S) respectively at the velocity v=1200m/sec (30MeV), and in the ratio singlet/triplet, which increases from 0.5 to 3.8 for velocity decreasing from 4500m/sec to 1200m/sec, but also in the different shapes of the associative and Penning cross sections for He(23S) and He(21S)